The present invention relates to fusion splicing of optical fibers. In particular, the present invention relates to an optical fiber fusion splice where at least one of the fibers has a controlled rate of mode field diameter (MFD) expansion to match the MFD of the other fused fiber.
Fusion splicing uses an electric arc to weld ends of two optical fibers together. The goal is to match the fibers to achieve the lowest possible signal loss. Extremely high temperatures (xcx9c2,000xc2x0 C.) are used to melt the silica glass ends of the fibers, which are then positioned together and allowed to cool and fuse. Single-mode fibers require precise alignment of the cores of the fibers. To minimize optical loss, it is desirable to match the MFD of the two fibers. Extremely sophisticated, computer-controlled equipment is used to align the fibers and monitor the optical match.
Erbium-doped fiber (EDF) is especially challenging to fusion splice because it requires a small core and high numerical aperture in order to maximize efficiency, thereby decreasing MFD. This issue is further complicated by the differing diffusion rates of dopant ions in EDF and the fibers it would be spliced to, such as those found in Erbium-doped fiber amplifier (EDFA) pump combiners.
Previous methods applied to EDF splicing involved intentionally expanding the MFD of the lower MFD fiber, by virtue of diffusion of index-raising ions out of the core of the fiber, to match the MFD of the second fiber. Index raising ion diffusion has the dual effect of lowering the refractive index of the core while simultaneously increasing the effective core size.
However, while the MFD expansion is desirable, the rate at which it occurs often proves to make control of it difficult. The dual effect of lowering index while increasing core size makes it extremely hard to stop the MFD expansion process at the correct time. For example, when an amplifier manufacturer utilizes EDF they may have problems with fusion times that are adequate for MFD expansion being too short to give adequate mechanical strength. Furthermore, short fusion times make it difficult to control tolerances of the fusion splicer, leading to problems with repeatability.
The matching problem becomes even more complicated as changes in the splicer due to buildup of silica on the electrodes (changes in fusion current, arc position, arc stability) have a greater effect because there is less fusion time for the instabilities to be averaged out.
Many of the previous efforts have focused on attempts to increase the expansion of a fiber core (and thus the MFD rather than slow it. One example discloses a design that utilizes fluorine diffusion into the core of a fiber in order to decrease the core""s index difference relative to the cladding, thereby increasing the MFD expansion rate. Another related example describes a method of intentionally diffusing index-raising core dopants to increase MFD to optimize splice loss.
The need remains for a regularly MFD-matched fusion splice and a repeatable method for achieving such a splice.
The present invention is directed to an optical fusion splice including an optical fiber having a fluorine ring between the core and the cladding. The fusion splice of the present invention, while allowing MFD expansion, gives the benefit of higher control over the process via the slowing of MFD expansion. Moreover, since the resulting curve of the rate of MFD expansion is predictable and may be controlled by the chemical composition of the fluorine containing fiber, the present invention allows the user to match the desired fusion time ranges with the desired MFD expansion rates.
A fusion splice in accordance with the present invention includes a first optical fiber having a first MFD and a first MFD expansion rate. The splice further includes a second fiber having a second MFD and a second MFD expansion rate, wherein the second MFD is lower than the first MFD. The first fiber may have a lesser concentration of index-raising dopants in the core than the second fiber or the first fiber may have a lower diffusivity of index-raising dopants in the core than the second fiber. The second rate of MFD change of the second fiber may be controlled by the amount of fluorine in the zone.
The second fiber comprises a core, a cladding, radially surrounding the core, and a zone of high-concentration of fluorine between the core and the cladding. High concentration is defined as when the fluorine concentration in the zone is greater than the fluorine concentration in either the core or the cladding. In a particular exemplary embodiment the maximum concentration of fluorine in the zone is between 0.5 to 6 mol %. The rate of MFD expansion of the first fiber is less than the rate of MFD expansion of the second fiber during the fusion splicing operation.
The core of the second fiber may be erbium doped, Al doped and/or La doped. The second fiber may also further comprise at least one diffusion barrier layer.
Likewise, the first fiber also may include a core, a cladding, and a zone of high-concentration of fluorine intermediate the core and the cladding, wherein the rate of MFD change of the first fiber is controlled by the amount of fluorine and the amount of index raising dopants.
A variety of devices may benefit from a fusion splice according to the present invention. A broadband amplifier including the fusion splice is contemplated wherein the first and the second fibers provide amplification for different wavelengths. Similarly, a broadband amplifier including the fusion splice, where only the second fiber provides amplification. The first fiber may be a pump laser combiner or a pump laser pigtail. These devices may be part of telecommunication systems.
A method for fusion splicing a first and a second optical fiber in accordance with the present invention includes the steps of providing a first fiber having a first MFD and a first MFD expansion rate upon heating and providing a second fiber having a second MFD and a second MFD expansion rate. The second MFD is less than the first MFD, and the second MFD expansion rate is greater than the first MFD expansion rate. The second fiber comprises a core, a cladding radially surrounding the core, and a high concentration F-ring intermediate the core and the cladding. The average concentration of F in the ring is higher than that of the center of the core or the outer edge of the cladding. The F-ring reduces the rate of MFD expansion of the second fiber when compared to a similar fiber without the F-ring.
The two fibers are then fused by applying heat to the end faces of each fiber and bringing them into close contact and optical alignment, while matching the MFD of the first and second fibers. The MFD of both the first and the second fiber may be monitored during the step of fusing, wherein the step of applying heat is controlled to achieve MFD matching. Alternatively, where the rates of MFD expansion of the first and second fiber are known, heat may be applied for a predetermined time and at a predetermined temperature, known to be the intersection of the first and second MFD expansion curves.